Electroluminescent thin film phosphors of various emission colors can be prepared by activating a zinc sulfide film with an appropriate rare earth element. Terbium is a frequently used rare earth since, in a zinc sulfide film, it can produce a high brightness green emission. Samarium in zinc sulfide produces a potentially useful red emission while other rare earths can yield various other colors. These rare earth elements are normally stable in the trivalent state. They are therefore not readily accommodated as direct substituents for divalent zinc in zinc sulfide. It is necessary to provide some form of charge compensation and this is desirably done by putting in an additional additive, often called a coactivator. Thus, in evaporative deposition methods, the usual source for rare earth elements to be used as activators in zinc sulfide films is the fluoride (e.g., TbF.sub.3) since then enough fluorine can be taken into the film structure to satisfy the trivalent bonding requirement of the rare earth. In this case, fluorine serves as the coactivator. It is also possible to use the rare earth element itself as the source with no coactivator. In this case, it is probable that the deviation in the valence of the rare earth is accommodated by zinc vacancies or other structural defects. Such films are generally less bright and less uniform electroluminescent emitters than those prepared using the fluoride source. They also tend to have poor dielectric properties.
In the case of sputtered zinc sulfide films, the situation is similar. Terbium can be added by sputtering simultaneously either from terbium fluoride and zinc sulfide targets or from terbium and zinc sulfide targets. As in the case of films prepared by evaporative methods, the emission is brighter and more uniform and the dielectric properties better in films prepared using the fluoride target. The same is true of other rare earths used as additives to zinc sulfide. A disadvantage in the use of such fluoride targets, however, is that they cannot be melted and cast but must be pressed from a powder. They are typically quite fragile and subject to easy breakage. It is therefore desirable to find alternative ways of adding coactivator during film deposition by sputtering techniques such that the rare earth itself can be used in the form of a metallic target.